The invention relates to a method of encoding audio and/or video signals for transmissions via a transmission medium. More particularly, the transmission medium is preferably an optically readable disc. Nevertheless, the transmission medium may also be a magnetic tape or disc, or a direct connection between a transmitter and a receiver. The invention also relates to a transmission medium on which audio and/or video signals are recorded, to an encoding apparatus for encoding and transmitting the audio and/or video signals, and to a decoding apparatus for receiving those signals after unloading and transmission.
The interactive compact disc (CDI) on which data for text and graphic images and digital audio is stored has been developed in the last few years. The presentation possibilities of CDI are considerably enhanced if full motion video pictures are also recorded on it. The known analog recording of a video signal on compact disc video (CDV) is not suitable for this purpose. For this reason, a video signal on CDI is digitized.
A full-motion video scene is considered as a sequence of pictures (of that video scene) of which there are, for example, 25 or 30 per second. Each picture comprises, for example, 256 picture lines and 352 pixels per picture line. The sequence of pictures is converted by means of a suitably chosen encoding method into a series of video blocks, each comprising so much digital information that each picture can be reconstructed without any noticeable loss of quality. Together with the audio signal and further data, the encoded video signal is recorded optically. A CDI may comprise various video scenes.
The most efficient encoding methods convert successive signal portions into successive code blocks of variable lengths. In the case of a video signal, these signal portions are formed, for example, by the pictures or picture pairs of which the signal is composed. Some pictures may be subjected to intraframe coding and are then converted into code blocks from which the picture can be reconstructed completely. Other pictures may be subjected to interframe coding, which means that the pictures can only be reconstructed with the aid of previous pictures. The code blocks of a video signal will hereinafter be referred to as video blocks. Due to their variable length, the successive video blocks are read at irregular instants when a disc in which they are stored is being played. Moreover, the video blocks on the disc may alternate with (or may even by interrupted by) other data signals, for example, a lip-synchronous digital audio signal corresponding to the video scene.
The pictures corresponding to the video blocks should be displayed at a constant frequency of, for example, 25 frames per second. However, the instant when a video block of the disc is being read hardly ever corresponds exactly to the instant when the corresponding picture from the video scene is to be displayed. In a player, the video blocks are, therefore, applied to a memory buffer at a frequency which is entirely determined the way in which they are "packed" on the disc. Subsequently, the video block are read from the buffer at the picture frequency required for display. On an average, the number of video blocks applied per second from the disc to the buffer equals the number of pictures displayed per second. The video block corresponding to the next picture to be displayed will always have to be stored completely in the buffer. Moreover, the buffer will already comprise a subsequent video block, a portion thereof or even a number of subsequent video blocks. As soon as a picture has been decoded, the corresponding video block may be removed from the buffer. The buffer space then released is written by one or more subsequent video blocks or a portion thereof. The number of video blocks stored in the buffer for later display is thus variable, and is greatly dependent on the encoding efficiency and the presence of signals other than the relevant video signal on the disc.
It would seem that the display of a video scene can start as soon as the first video block has been received completely and is stored in the buffer. However, this is not the case. It is possible that a subsequent video block is too large to be scanned (and stored in the buffer) within the required time (1/25 sec) to (decode and) display the picture corresponding to the first video blank. As a result, the corresponding picture of the subsequent video block cannot be displayed in time. The absence of a complete video block in the buffer at the instant when the corresponding picture has to be decoded and displayed is sometimes referred to as underflow of the buffer.
Underflow of the buffer also occurs if a large quantity of other (non-video) data has been packed together with the video blocks of a scene causing the buffer to be temporarily not filled with video blocks. Because of that other data the buffer empties, and at a given instant the video block for the corresponding next picture to be displayed is not yet present. The display of the video scene then stalls, and the pictures are not smooth moving.
If the display of a picture of a video scene starts too late after the-corresponding video block has been received, it is probable that the buffer will fill so that the display of the video scene also stalls. This is referred to as overflow of the buffer.
The same problems occur with audio signals which may also be recorded on the disc in a non-contiguous manner.